1. Introduction
This invention is directed to a process for the waste treatment of metal plating solutions containing active complexing agents.
2. Description of the Prior Art
Solutions capable of plating metal are well known in the art. The most commonly used metal plating solutions comprise electrolytic solutions and electroless solutions--i.e., those capable of plating metal without the need for electricity.
The most frequently used electroless solutions for plating metal comprise copper and nickel plating solutions. Such solutions generally comprise four major ingredients dissolved in water. These ingredients include (1) a source of the metal plating ions, (2) one or more complexing agents capable of maintaining said metal ions in solution, (3) a reducing agent capable of reducing said metal ions to metallic form in the presence of a catalytic surface, and (4) a pH adjustor to maintain solution pH within a desired range. Other conventional additives include stabilizers to prevent the solution from undergoing spontaneous decomposition, brighteners to improve appearance and exaltants or rate retardants to regulate plating rate.
Electroplating solutions are used to plate a variety of metals and bear some resemblance to electroless plating solutions. However, they typically do not require reducing agents for plating and many do not require complexing agents. Many of those solutions that do use complexing agents are intended for use at an alkaline pH. Electroplating solutions also contain other additives such as brighteners and grain refiners, though those specific materials used to perform such functions in electroless baths are not necessarily useful for the same function in an electrolytic bath.
The major ingredients of plating solutions are well known. The metal ions for an electroless solution are derived from a salt such as a sulphate or chloride. A common source of a metal for an electroplating solution is a sulfamate. The reducing agent for electroless copper plating solutions is generally formaldehyde and for electroless nickel plating solutions, sodium hypophosphite. Other reducing agents such as hydrazine and various boron compounds are also used, but less frequently.
There are a wide variety of complexing agents known for metal plating solutions. These include various amines such as primary, secondary, tertiary and quaternary amines, various carboxylic acids and various amino acids. Illustrative examples of complexing agents used in such solutions comprise Rochelle salts (a double salt of sodium and potassium tartrate), pentahydroxypropyl dithylene triamine and ethylene diamine tetracetic acid.
Representative electroless nickel and copper plating solutions are disclosed in U.S. Pat. Nos. 3,329,512; 3,383,224; 3,650,777; 3,674,516; 3,915,716; and 4,036,651, all of which disclose the compositions of known plating solutions. The contents of these patents are incorporated herein by reference. Representative electrolytic metal plating solutions are disclosed in the Metal Finishing Guidebook and Directory, Metals and Plastics Publications, Inc. Hackensack, N.J. 1976, pp. 177 to 338. The contents of this portion of the handbook are also incorporated herein by reference.
It is known in the art that when metal plating solutions are used, the content of various components of the solution are consumed. Electroless metal plating solutions deposit metal by contact of the complexed metal in solution with a catalytic surface in the presence of a reducing agent. As metal plating proceeds, the concentration of both the metal in solution and the reducing agent is reduced, the metal by plate-out onto a substrate and the reducing agent as a consequence of consumption in accordance with the chemical reaction controlling plating.
It is also known that metal plating solutions can be replenished by addition or replenishment of consumed ingredients. Thus, when approximately from twenty to forty percent of the metal originally contained in an electroless solution is consumed, it is conventional to add a replenisher formulation consisting primarily of the metal salt and reducing agent and also minor quantities of other ingredients lost by drag-out or otherwise. Though the useful life of a plating solution may be prolonged by replenishment, eventually the solution becomes unsatisfactory and disposal is necessary. In addition, growth of metal plating solutions as a consequence of replenishment with replenisher in solution form also requires the disposal of excess solution. Further, rinse waters for plated parts build up metal values and these rinse waters also must be dumped. Frequently, for disposal, the metal plating solution, the growth resulting from replenishment and the rinse waters are combined for both waste treatment and disposal. Consequently, for purposes of definition, the term plating solution is defined broadly to include solutions containing dissolved metals and active complexing agent whether derived from the plating solution, rinse waters or growth or any combination of solutions resulting from a plating line.
The composition of several typical metal plating solutions, at initial make-up and when spent, are set forth below for purposes of illustration. The concentrations of the components of the spent solution are given as a range because they can vary within very broad limits dependent upon the source of the solution as discussed above. Hence, the ranges given should be viewed as illustrative only and should not be interpreted as limitations to the scope of the invention.